1. Field of the Invention
This invention relates to a semiconductor device such as a field effect transistor, etc., more particularly to a semiconductor device of a main part is constituted of a polycrystalline thin film semiconductor layer containing germanium.
2. Description of the Prior Art
Recently, for providing a scanning circuit portion of an image reading device for use in image reading, such as a one-dimensional photosensor made in a continuous length or a two-dimensional photosensor of an enlarged area, or for providing a driving circuit of an image display device utilizing liquid crystal (abbreviated as LC), electrochromic material (abbreviated as EC), it has been proposed to form a field effect thin film transistor by using as the base material a silicon thin film formed on a certain substrate, corresponding to scale-up of these devices. And, in the prior art, while the effective carrier mobility (hereinafter written as .mu.eff) required by a scanning circuit portion of a high function reading device or a driving circuit portion of an image display device is about 50 to 100 cm.sup.2 /V.sec, .mu.eff of the thin film transistor (TFT) by use of an amorphous silicon thin film is as small as 0.1 cm.sup.2 /V.sec, and therefore it was not necessarily suitable for providing the above circuit portion. On the other hand, a polycrystalline silicon thin film has a mobility .mu.eff greater than an amorphous silicon thin film, but in order to respond to the above requirement, an annealing step is required, whereby such problems that the steps become complicated or that no uniform film over a large area could be obtained were involved.
On the other hand, formation of a polycrystalline germanium thin film has been attempted in the prior art according to the vacuum vapor deposition method. The Hall mobility (hereinafter written as .mu.H) of the film obtained by this method is extremely large, as great as some 100 cm.sup.2 /V.sec, and its .mu.eff was also expected to be large. However, in a non-doped polycrystalline germanium thin film there is generally formed a high density acceptor level, and therefore, doping efficiency of an impurity to make a n-type or p-type semiconductor was poor. For this reason, no polycrystalline germanium thin film semiconductor element has been practically used. In other words, because the so called intrinsic semiconductor can be formed only with difficulty, doping efficiency into germanium matrix by addition of an impurity was very bad. Also, in a germanium thin film, there is observed a phenomenon of Thermal Conversion in which conversion from n-type semiconductor to p-type semiconductor occurs by heat treatment, and therefore it was not suitable for device preparation including a heat treatment step. Thus, under the present situation, an element or a device by using as the base material a polycrystalline germanium thin film could not sufficiently exhibit desired characteristics or reliability.
Also, due to smaller energy gap of germanium as compared with silicon, there is involved a drawback of, for example, greater reverse direction saturation current, which may sometimes cause troubles in practical application. Further, as another disadvantage, due to smaller energy gap, the concentration of the carrier raised by heat energy from the valence electron band to the conduction band approaches the concentration of the carrier caused by an impurity at a low temperature, whereby the temperature tolerance range of the device was narrow.